Sheet feed assembly defining curved and straight feed path

ABSTRACT

A sheet feed assembly for feeding sheets of material having a known bending stiffness through a device with low friction. The sheet feeding assembly has feed path structures for defining a feed path, the feed path having a first point with a first radius of curvature, a second point with a second radius of curvature and a transition section extending between the first point and the second point. The transition section defines conforms to a shape adopted by one of the sheets of material extending from the first point where it has a curvature of the first radius, to the second point where it has a curvature of the second radius.

FIELD OF THE INVENTION

The present invention relates to feeding sheets of material along a feedpath that has curved sections that transition into a straight section orother, differently curved sections. In particular, the invention relatesto feeding sheets of paper through an inkjet printer.

BACKGROUND OF THE INVENTION

At first glance, a observer may conclude that there is relative freedomin the shape a paper feed path may take, however there are some generalrules which must be obeyed for a sheet of paper to be able to conform toa paper guide. If the paper cannot conform to the shape of the guide, itis not actually being “guided”. This can lead to undesirable printartefacts.

First (and most obviously): the path must not have any step jumps in it.Even minute steps cause the paper to jam during threading and can causeprint artefacts as the trailing edge flicks on passing. The paper cannotconform to a step jump. In mathematical terms, the path must becontinuously differentiable.

Second: At all points along the curve the slopes (tangents to the path)must match. This means the path cannot have sharp kinks in it where theradius of curvature is essentially zero. Again, quite clearly, the papercannot conform to a sharp bend. Mathematically this means the firstdifferential (tangents) at the transition points from one type of curveto another must also be equal.

Third (and not so obvious): At any point along the path, theinstantaneous radii of curvature must not have step jumps, or the paperwill not conform to the guide.

This is illustrated by the general bending equation:

R=EI/M

Where: R is the radius of curvature

-   -   E is the materials bending modulus    -   I is the second moment of area    -   M is the applied bending moment

A step jump in radius of curvature requires a step jump in M, thebending moment, since the other variables are fixed properties of themedia. However, it is not physically possible to apply such a step inthe moment. In reality, the path has a smoothly varying instantaneousradius of curvature at all points. For example, a guide with an arc ofconstant curvature leading to a tangential straight part, has a constantarc portion, where R is constant and finite, and consequently M isconstant and finite. In the straight portion, R is infinite, so thebending moment M=0. However at the transition point both conditions mustexist simultaneously. This is a contradictory condition and, as aresult, the paper cannot stay in contact with the guide at such a point.

Many fields of industry require sheet material to be moved along a feedpath. If the feed path is curved, it is exceedingly difficult to bendthe extreme leading edge into the curved shape. As the leading edgecontacts the guides or rollers that define the curve in the feed path,the contact force acting at the edge bends the sheet because of thebending moment the force creates. However, at a distance extremely closeto the edge, the moment arm (i.e. the distance to the point where forceis applied) is not long enough to generate the bending moment necessaryfor the sheet to flex into the curved shape defined by the guides. Atdistance infinitesimally close to the leading edge, the moment arm isinfinitesimally small and the sheet is in fact flat; not curved at all.Hence there is a relatively flatter leading edge as the sheet is fedaround the defined curve. This gives the leading edge a tendency to‘chisel’ into the guide surface because it is not conforming to thecurve. The chiselling action increases friction and can be the drivingmechanism behind feed jams. Feeding sheets of paper through a printer isan example where jams, or ‘paper cockle’, in the sheet feed system are acommon problem.

Often it will be necessary for the sheet feed path to be curved andsubsequently transition into a straight line. At these points oftransition in the feed path, the sheet material is prone to deviate fromthe nominal or ideal path. As discussed above, this is particularly soof the leading and trailing edges of the sheet. The stiffness in thesheet causes it to deviate from the feed path until the leading edgeenters the nip of a downstream roller pair (or guide surface) while thetrailing edge can spring away from the feed path once it is releasedfrom between a roller pair.

If the sheet is subject to a surface treatment such as the coatings onhigh quality photo papers, this deflection from the nominal feed pathcan be especially detrimental. Inkjet printing requires the mediasubstrate to stay on the feed path for optimum print quality. If theleading or trailing edge of the media sheet deviates from the feed path,then the distance from the nozzles to the surface of the sheet willchange. Varying the flight time of the ink droplets will result invisible artefacts in the resulting print.

The invention is well suited to paper feed assemblies in inkjetprinters. In light of the wide spread use of inkjet printers, theinvention will be described with reference to this particularapplication. However the ordinary worker will appreciate that theinvention is equally relevant to other applications involving sheet feedmechanisms and the broad inventive concept is not restricted to thefield of inkjet printers.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a sheet feedassembly for feeding sheets of material having a known bendingstiffness, the sheet feeding assembly comprising:

feed path structures for defining a feed path, the feed path having acurved section connected to a straight section such that the straightsection is downstream of the curved section with respect to the sheetfeed direction; wherein,

the curved section and the straight section meet at a transition pointwhere the straight section is tangential to the curved section andconfigured such that a sheet partially in the curved section andpartially in the straight section has zero bending moment at thetransition point and zero bending moment in any part on the straightportion.

By transitioning to the straight section of the feed path at a pointwhere the sheet inherently has zero bending moment as it passes alongthe feed path, the leading edge and the trailing edge has no drivingmechanism to deviate from the paper path. The unconstrained leading andtrailing edges follow the straight feed path like the constrainedintermediate portions of the sheet. In this way, the printing gapbetween the nozzles and the surface of the media sheet remains constant.

Preferably, the feed path structures include a roller pair positionedsuch that its nip is at the transition point. Preferably, the feed pathstructures include a roller partially defining the curved section of thefeed path.

Optionally, the curved section has an upstream end where the feeddirection is parallel to and opposite the feed direction at thetransition point. Optionally, the feed structures include a chuteextending between the upstream end and the transition point, the chutehaving an inner curved surface nesting within an outer curved surface todefine a gap between the inner and outer curved surfaces through whichthe sheets are fed. Optionally, the inner and the outer curved surfacesare identical and displaced from each other to form the gap. Preferably,the inner and outer curves are identical to a bending curve adopted byat least part of a sheet that is bent over and held by co-planar anddirectly opposing forces such that tangents at both ends of the bendingcurve are parallel to each other and spaced apart by the spacing betweenthe upstream end and the transition point.

Preferably, the feed path extends through an inkjet printer. In afurther preferred form, the inkjet printer has a printhead positionedadjacent the feed path downstream of the transition point. In aparticularly preferred form, the upstream end of the chute receivessheets sequentially fed from a stack of the sheets by a picker arm.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments will now be described by way of example only toillustrate the present invention, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of a sheet feed assembly in aninkjet printer according to the prior art;

FIG. 2 is a diagram of a cantilevered simple beam deflecting under aload applied to its end;

FIG. 3 is a schematic representation of a sheet feed assembly for aninkjet printer with rollers positioned using the simple beam analysis ofFIG. 2;

FIG. 4 is a diagram of a sheet being bowed by co-planar, opposing forcesuntil its ends are parallel;

FIG. 5A is a plot of a curve conforming to the shape of the bowed sheetin FIG. 4;

FIG. 5B is an enlarged diagram of the incremental secant line, dy, dxand θ at a point on the curve plotted in FIG. 5A;

FIG. 6 is a plot of a curve conforming to the shape of a sheet bowed byco-planar, opposing forces with nonparallel ends;

FIG. 7A is schematic representation of a C-chute in accordance with thecurve plotted in FIG. 6;

FIG. 7B is an enlarged diagram of the resolution of the force acting atthe feed roller pairs into normal and tangential components;

FIG. 8 is a section view of an inkjet printer incorporating a C-chuteshaped to conform to the curve plotted in FIG. 5A;

FIG. 9 is an exploded perspective of the printhead cartridge used in theprinter of FIG. 8;

FIG. 10 is a section view of the print engine used in the printer ofFIG. 8; and,

FIG. 11 is a schematic representation of a sheet feed assembly accordingto the invention with a curved chute shaped to correspond to the curveplotted in FIG. 5A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Sheet Feed ChuteConforming to Shape of Bowed Sheet

Referring to FIG. 1, a sheet feed assembly is shown feeding a sheet 26of media substrate past a printhead 2. The feed path 8 is extends aroundan idler roller 14 and through the nip 16 of input drive rollers 4 and6. The input drive rollers drive the sheet 26 past the printhead 2 andinto the nip 18 of the exit rollers 22 and 24.

The inherent bending stiffness in the sheet 26 causes the leading edgeto deviate away from the feed path 8 as it leaves the idler roller 14and the guiding shroud 12. The input drive rollers 4 and 6 draw thesheet into the nip 16 and therefore back on to the feed path 8. However,the input drive rollers 4 and 6 are mounted so that the opposing pinchforce from each roller are normal to the straight part of the feed path8. This does nothing to redirect the sheet back to the feed path.

The leading edge of the sheet 26 continues to deviate as it crosses theprintzone 20 of the feed path 8. The printing gap between the printhead2 nozzles and the feed path 8 is X. The printing gap between the nozzleson the printhead 2 and the leading edge is X′—significantly smaller thanX. Therefore the flight time of the droplets onto the leading edge willbe shorter than the droplet flight time once the sheet 26 enters the nip18 of the output rollers 22 and 24, and draws the sheet back to the feedpath 8. The variation of droplet flight times affects dot spacing on theprinted sheet resulting in visible artifacts in the print.

Referring to FIG. 2, the sheet 26 has been modeled as a simplecantilever beam loaded at its end (or a distance L from the fixed end).The radius of curvature changes along the beam 26 until it reachesinfinity at the free end. That is, the beam is flat at the very end, asthe load F has no moment arm to bend it. If the beam 26 were to extendbeyond the point where F is applied, it would follow the tangent 8(again, no moment to bend it).

The radius R of the curvature at any point on the beam 26 can becalculated using:

R=EI/M

where:

-   R is the radius of curvature of the beam at any given point along    its length;-   E is Young's modulus of the sheet material;-   M is the ending moment at that point on the beam; and,-   I is the second moment of area about an axis across one surface of    the sheet.

Using this model, it is also possible to determine T, the distancebetween the intersection of the tangent 8 on the wall 28 and the centreof radius R, and the angle θ between the wall and a normal to thetangent 8.

Referring to FIG. 3, the cantilever beam model of FIG. 2 is used toconfigure the feed path structures. The roller 14 has a radius R equalto that calculated in the beam model. The tangent line 8 becomes theflat section of the feed path extending past the printhead 2. L, T and θare used to position the centre of the roller 14 and the nip 16 betweenthe input drive rollers 4 and 6. As the sheet 26 is fed through theinput drive rollers 4 and 6, it has no bending moment at that point, andno bending moment at any point downstream (with respect to feeddirection 10). Accordingly, the sheet 26 inherently follows the flatfeed path 8.

The output rollers (not shown) and downstream feed path structures (notshown) can be similarly positioned relative to each other to avoid thetrailing edge from flicking up or down when it is released from theinput drive rollers 4 and 6.

FIG. 4 is a sketch of a sheet 26 bowed by coplanar, opposing forces Funtil the ends 30 are parallel to each other. The shape of the bowedsheet 26 can be determined iteratively using the three equations set outbelow. Using the shape provided by this model, it is possible to form atheoretically frictionless C-shaped chute. The chute is theoreticallyfrictionless because it dresses to exactly the same shape as the bowedsheet and therefore, there is no normal force at any points of contactbetween the sheet and the chute surface.

Curvature of Bowed Sheet

Referring to FIG. 5A, the curve of the sheet 30 is shown with its axisof symmetry (corresponding to the surface of the wall 28 shown in FIG.4) on the X axis. Angle θ is scribed between R (the radius of curvatureat any point) and the horizontal. Force F is applied by rollers at thepoint 100 that the sheet transitions from a curved to a straight feedpath. At this point on the curve, x_(max) is found analytically (whereθ=90°). As best shown in FIG. 5B, ds is the secant line at a point onthe curve for given θ.

Find x_(max):

${s} = {\frac{x}{\sin \; \theta} = {\frac{EI}{F}\frac{\theta}{\left( {x_{\max} - x} \right)}}}$${\left( {x_{\max} - x} \right){x}} = {{{\frac{EI}{F}\sin \; \theta {\theta}}\therefore{\int_{0}^{x_{\max}}{\left( {x_{\max} - x} \right)\ {x}}}} = {\frac{EI}{F}{\int_{0}^{\frac{\pi}{2}}{\sin \; \theta \ {\theta}}}}}$${{\int_{0}^{x_{\max}}{x_{\max}{x}}} - {\int_{0}^{x_{\max}}{x{x}}}} = {- {\frac{EI}{F}\left\lbrack {\cos \; \theta} \right\rbrack}_{0}^{\frac{\pi}{2}}}$${x_{\max}^{2} - \left\lbrack \frac{x^{2}}{2} \right\rbrack_{0}^{x_{\max}}} = \frac{EI}{F}$${x_{\max}^{2} - \frac{x_{\max}^{2}}{2}} = {\frac{x_{\max}^{2}}{2} = {{\frac{EI}{F}\therefore x_{\max}} = \sqrt{2\frac{EI}{F}}}}$

Numeric Solution for the Complete Shape of the Proposed Curve:

Using the above equation: for a given input value of x_(max), we cansolve

$\frac{EI}{F}$

Then from:

s = Rθ $R = \frac{EI}{M}$ M = F(x_(max) − x)${1.\;\therefore{s}} = {\frac{EI}{F}\frac{\theta}{\left( {x_{\max} - x} \right)}}$2.  x = sin  θ × s${3.\mspace{14mu} {y}} = \left\lbrack {{s^{2}} - {x^{2}}} \right\rbrack^{\frac{1}{2}}$

By iterating 1 □ 2 □ 3 in a computational loop and vector summation, wecan produce the correctly shaped curve. Knowing the correct shape for agiven x_(max) may not be sufficient, since it is usually important thatthe distance 2 y_(max) between the curves tips matches into the pathsystem. We can solve this problem using a “shooting” method. We can do abinary search to iterate x_(max) and rerun the algorithm to find thevalue of 2y_(max) for the correct curve to fit the design boundaryconditions.

Also of interest is the minimum radius of curvature of this shape,because it suggests when the media will retain a permanent set:

R₀ when x=0 i.e. the minimum radius of the curve and the maximum bendingmoment. Referring again to FIG. 5A, the first boundary condition 98 isx=0, where R=R₀ and θ=0. The second boundary condition 100 is x=x_(max),R is infinite and θ=90°. Between these boundary conditions, the curvedfeed path is a transition section 102 along which the radius varies inaccordance with a sheet of material constrained at those boundaryconditions.

Non-Parallel Entry and Exit Paths

In some situations, the feed path does not turn the sheet through a full180 degrees. Referring to FIG. 7A, the ends 30 of the sheet 26 are notparallel. The first and second feed rollers 50 and 52 hold the ends 30at an angle to each other and exert a buckling force F on the sheet 26.The C-chute inner surface 34 and the outer surface 32 conform to thebuckled shape of the sheet 26 and if the rotation of the first andsecond feed rollers 50 and 52 is synchronized, there is theoretically nofriction between the sheet 26 and the chute. This requires close controlof the feed rollers 50 and 52 such that the co-linear, opposing bucklingforces F can each be resolved (see FIG. 5B) into a force F_(n) actingnormal to the sheet and F_(s) acting parallel to the plane of the ends30. If the magnitude of F_(s) is the same at each of the feed rollers 50and 52, the sheet 26 does not scrub against the inner or outer surface34, 32 of the C-chute 54.

As shown in FIG. 6, the numeric calculation method for determining thecurve of a buckled sheet is the same as for a sheet buckled until itsends 30 are parallel (see FIG. 4) except the boundary condition becomesθ<90°.

The same equations and method for the numeric solution described abovestill hold true, but the analytic solution to solve for x_(max) becomes:

$M = {{F\left( {x_{\max} - x} \right)} = \frac{EI}{R}}$$\frac{EI}{RF} = \left( {x_{\max} - x} \right)$

where:

-   R is the radius of curvature of the sheet at any given point along    its length;-   E is Young's modulus of the sheet material;-   M is the ending moment at that point in the sheet; and,-   I is the second moment of area about an axis across one surface of    the sheet.

$\frac{{EI}\; \sin \; \theta {\theta}}{F} = {\left( {x_{\max} - x} \right){x}}$x = sin  θ ⋅ s R ⋅ θ = s$R = \frac{x}{\sin \; \theta {\; \theta}}$

Iterating through equations 1→2→3 set out above in a computational loopand then vector summating:

$x_{\max} = \sqrt{\frac{2{EI}}{F}\left\lbrack {1 - {\cos \; \theta_{\max}}} \right\rbrack}$

Interestingly:

R₀ when x=0 i.e. the minimum radius of the curve and the maximum bendingmoment

${\therefore M_{0}} = {{F\left( {x_{\max} - x} \right)} = {{{Fx}_{\max}\therefore R_{0}} = {\frac{EI}{M_{0}} = {\frac{EI}{{Fx}_{\max}} = {\frac{x_{\max}^{2}}{2x_{\max}} = \frac{x_{\max}}{2}}}}}}$

Hence, the circle of minimum radius R₀ has a diameter=x_(max).

Sheet Feed for High Speed Printer

A C-chute is useful in an inkjet printer to create a paper path betweena feed tray at the base of the printer and a collection tray formed bythe top surface the printer. This is a compact configuration with asmall footprint. FIG. 8 is a section view of a printer 66 with thisconfiguration. This printer uses a print engine shown in copending U.S.Ser. No. 12/014772 (Our Docket RRE017US), the contents of which areincorporated herein by cross reference. The print engine of a printerrefers to the key mechanical structures of an inkjet printer. Theperipheral structures such as the outer casing, the paper feed system,paper feed and collection trays and so on are configured to suit thespecific printing requirements of the printer (for example, photoprinter, network printer or SOHO printer). The printer shown in FIG. 8is an A4 SOHO printer.

FIG. 10 shows a section view of the print engine 3 with a sheet of media26 extending past the printhead integrated circuit (IC) 64. Theprinthead 2 is in the form of a removable printhead cartridge 70. FIG. 9is an exploded perspective of the printhead cartridge 70 showing the topmolding 72 with a central web 74 for structural stiffness and to providetextured grip surfaces 76 for manipulating the cartridge duringinsertion and removal. Ink from the ink tanks 56 (see FIG. 8) is fed tothe inlet manifold 82. The inlet manifold has five inlet ink spouts 88set in an inlet shroud 78. Each of the inlet spouts 88 feed a respectivelongitudinally extending channel (not shown) in the liquid crystalpolymer (LCP) molding 92. Air cavities 94 above the channels damp anyhydraulic hammer in the ink when printing stops abruptly. A series ofprinthead integrated circuits (IC's) 64 are mounted to the underside ofthe LCP molding 92. The printhead IC's 64 define an array of inkejection nozzles (not shown) that extend the width of the sheets 26 tobe printed. Hence, the printer is a pagewidth printhead that remainsstationary in the printer during printing.

At the downstream end of the LCP molding 92 is the outlet manifold 84.It has five outlet ink spouts 90, each fluidically connected to one ofthe longitudinally extending ink channels respectively. The outletshroud 80 is configured to allow the outlet spouts 90 to engage anoutlet interface 96 (see FIG. 10) which feed to a sump 86 (see FIG. 8).The sump 86 is used when the printer fluidic system actively primes ordeprimes the printhead 2. Detailed description of the fluidic system isprovides in the Applicant's U.S. Ser. No. 11/872719 (our docketSBF009US) the contents of which is incorporated herein by reference.

In the interests of clarity, FIG. 11 is sketch of the printer 66 showingthe operation of the C-chute 54 in relation to the straight feed path 8in the print zone 20. The C-chute 54 has an inner surface 34 and anouter surface 32. The geometries of the inner surface and the outersurface are the same with the exception of the upstream and downstreamend portions where the inner surface is reduced and or the outer surfaceis expanded to accommodate the thickness of the sheet and sometolerance. The majority of the gap between the inner and outer surfacesis due to displacement of the inner 34 relative to the outer 32 alongthe C's central line of symmetry.

In operation, paper sheets 26 are sequentially fed from the stack 40 inthe paper tray 38 by the picker arm 36 into the C-chute feed rollers 46and 48. The sheets 26 enter the C-chute 54 and the outer surface 32guides the leading edge around. The geometry of the outer surface 32 issuch that the leading edge easily feeds into and conforms to the curve.Contact forces acting at the leading edge to bend the sheet into thenecessary shape have a long lever arm to the point where the sheetcontacts the inner surface 34.

As discussed above, the feed path 8 at the entry and exit to the C-chuteis parallel. Hence, the leading edge does not deviate from the straightpath 8 as it is fed through input drive rollers 4 and 6. The sheetcontinues along the path 8 directly into the nip 18 of the outputrollers 22 and 24. The printed sheets 44 drop from the output rollersinto the collection tray 42.

Precise synchronization of the C-chute feed rollers 46, 48 and the inputdrive rollers 4 and 6, makes the chute theoretically frictionless. Thetwo roller pairs are feeding the sheet 26 in parallel but opposingdirections. The curvature of the sheet 26 between the roller pairs isthe curvature that the sheet wants to adopt naturally. Hence, there isno normal force component to any contact between the sheet and the inneror outer surface, and therefore no friction.

The invention has been described herein by way of example only. Theordinary worker will readily recognize many variations and modificationswhich do not depart from the spirit and scope of the broad inventiveconcept.

1. A sheet feed assembly for feeding sheets of material having a knownbending stiffness, the sheet feeding assembly comprising: feed pathstructures for defining a feed path, the feed path having a curvedsection connected to a straight section such that the straight sectionis downstream of the curved section with respect to the sheet feeddirection; wherein, the curved section and the straight section meet ata transition point where the straight section is tangential to thecurved section and configured such that a sheet partially in the curvedsection and partially in the straight section has zero bending moment atthe transition point and zero bending moment in any part on the straightportion.
 2. A sheet feed assembly according to claim 1 wherein the feedpath structures include a roller pair positioned such that its nip is atthe transition point.
 3. A sheet feed assembly according to claim 1wherein the feed path structures include a roller partially defining thecurved section of the feed path.
 4. A sheet feed assembly according toclaim 1 wherein the curved section has an upstream end where the feeddirection is parallel to and opposite the feed direction at thetransition point.
 5. A sheet feed assembly according to claim 4 whereinthe feed structures include a chute extending between the upstream endand the transition point, the chute having an inner curved surfacenesting within an outer curved surface to define a gap between the innerand outer curved surfaces through which the sheets are fed.
 6. A sheetfeed assembly according to claim 5 wherein the inner and the outercurved surfaces are identical and displaced from each other to form thegap.
 7. A sheet feed assembly according to claim 6 wherein the inner andouter curves are identical to a bending curve adopted by at least partof a sheet that is bent over and held by co-planar and directly opposingforces such that tangents at both ends of the bending curve are parallelto each other and spaced apart by the spacing between the upstream endand the transition point.
 8. A sheet feed assembly according to claim 7wherein the feed path extends through an inkjet printer.
 9. A sheet feedassembly according to claim 8 wherein the inkjet printer has a printheadpositioned adjacent the feed path downstream of the transition point.10. A sheet feed assembly according to claim 9 wherein the upstream endof the chute receives sheets sequentially fed from a stack of the sheetsby a picker arm.
 11. A sheet feed assembly for feeding sheets ofmaterial having a known bending stiffness, the sheet feeding assemblycomprising: feed path structures for defining a feed path, the feed pathhaving a first point with a first radius of curvature, a second pointwith a second radius of curvature and a transition section extendingbetween the first point and the second point; wherein, the transitionsection defines conforms to a shape adopted by one of the sheets ofmaterial extending from the first point where it has a curvature of thefirst radius, to the second point where it has a curvature of the secondradius.
 12. A sheet feed assembly according to claim 11 wherein the feedpath in the transition section is determined analytically using thefirst point and the first radius of curvature as a first boundarycondition and the second point and the second radius of curvature as asecond boundary condition.
 13. A sheet feed assembly according to claim11 further comprising a first sheet feed drive roller pair and a seconddrive roller pair for feeding the sheet along the feed path wherein thefirst drive roller pair and the second drive roller pair are configuredfor synchronous operation.
 14. A sheet feed assembly according to claim11 wherein the feed path at the first point is normal to the feed pathat the second point.
 15. A sheet feed assembly according to claim 14wherein the feed structures include a chute extending between the firstpoint and the second point, the chute having an inner curved surfacenesting within an outer curved surface to define a gap between the innerand outer curved surfaces through which the sheets are fed.
 16. A sheetfeed assembly according to claim 15 wherein the inner and the outercurved surfaces are identical and displaced from each other to form thegap.
 17. A sheet feed assembly according to claim 16 wherein the feedpath extends through an inkjet printer.
 18. A sheet feed assemblyaccording to claim 17 wherein the inkjet printer has a printheadpositioned adjacent the feed path downstream of the second point.